Lithographic printing presses typically use a lithographic printing plate that is mounted on a cylinder of the printing press. The lithographic plate carries a lithographic image on its surface. The lithographic image consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as oleophobic (or, hydrophilic i.e. water-accepting, ink-repelling) areas. In conventional (so-called “wet”) lithographic printing, ink as well as an aqueous solution are supplied to the lithographic plate. The ink is retained in the oleophilic areas and repelled in the oleophobic areas. The retained ink is transferred from the lithographic plate onto a receiver material, which is typically paper.
Most lithographic printing plates comprise a substrate having a radiation-sensitive coating applied thereto. These coatings can be selected to be sensitive to limited ranges of radiation including ultraviolet radiation, visible radiation or infrared radiation. Coating compositions are typically useful only within the limited range of wavelengths. The coating may react to the radiation such that the portions of the coating exposed to radiation become soluble in a developing solution. The solubilized portions are removed by soaking the exposed lithographic plate in a developing solution. Such plates are known as positive-working printing plates. On the other hand, the coating may react to the radiation such that the exposed areas become hardened and insoluble to developing solutions. The remaining unexposed, soluble portions are removed by soaking the exposed lithographic plate in a developing solution. Such plates are known as negative-working printing plates. Coating systems used in positive working plates typically are not useful in negative working plates and vice versa.
Positive working thermal printing plates useful within the infrared (IR) wavelength range typically employ a coating consisting of hydroxy substituted polymers, such as novolacs and/or resoles, and infrared (IR) absorbing dyes coated on a suitable lithographic substrate. When the lithographic plate is image-wise exposed to IR energy a solubility difference is generated such that the exposed areas show greater solubility in aqueous alkali developing solutions than the unexposed areas. Thus, after image-wise exposure to IR energy and development in an aqueous alkali developer, a positive image of oleophilic coating in the unexposed areas and hydrophilic lithographic substrate in the exposed areas is produced.
To enhance the solubility difference between exposed and unexposed areas additives, often referred to as solubility suppressors, are added to the coating. It has been theorized that the solubility suppressor generates secondary forces, such as hydrogen bonding, with the hydroxy groups on the polymer to further reduce the solubility of the coating in aqueous alkali developers. Upon image-wise exposure to IR energy, heat generated by the infrared absorbing dye disrupts these secondary forces restoring solubility in the aqueous alkali developer. No chemical bonds are formed between the solubility suppressor and the hydroxy substituted polymer and both the solubility suppressor and the hydroxy substituted polymer retain their chemical identity. One known class of solubility suppressing compounds is diazonaphthoquinone (DNQ) derivatives. These compounds are also sensitive to ultraviolet (UV) light. The mechanism through which UV exposure restores solubility to coatings containing DNQ's differs from the mechanism through which IR energy exposure restores solubility in that UV exposure of a coating containing DNQ causes chemical rearrangement of the DNQ. The chemically rearranged DNQ can no longer generate secondary forces with the hydroxy groups on the polymer and solubility of the coating in aqueous alkali developers is restored. The IR absorbing dye may also function as a solubility suppressing compound.
Despite the range of chemicals that function as solubility suppressors through secondary forces, presently known positive working thermal printing plates can be improved. On the one hand, plates can be produced which require high levels of IR energy exposure and/or extremely strong, difficult to control aqueous alkali developing solutions to ensure that all the coating is removed from the exposed areas, otherwise undesirable toning and tinting is encountered with these plates during the printing operation. On the other hand, plates can be produced which require lower levels of IR energy exposure and/or less strong, easier to control developing solutions to clean out the exposed area, but the unexposed areas are not sufficiently insolubilized to produce the required reproduction quality and/or number of printed copies. Additionally, lithographic plate latency is also of concern. Latency describes the amount of time that an exposed lithographic plate can be held before developing is necessary. Latency times vary depending on the coating system from less than 30 minutes to many hours. Maximum latency is desirable for lithographic plate coatings. A lithographic plate having improved processing requirements, reproduction quality, life and latency would be desirable.